1. Field of the Invention
The present invention relates, in general, to methods and systems of wireless networks for determining and negotiating channel bandwidths to be used within the network. More particularly, the invention relates to, but is not limited to, wireless networks operating in the Sub 1 GHz band, especially networks using the emerging IEEE standard 802.11 ah.
2. Relevant Background.
In the well-established IEEE 802.11a/b/g/n standards for wireless local area networks, an access point (AP) is a radio communication device that communicates with several other devices, called stations (STAs), such as computers, cellphones, printers, etc. The AP typically acts as a hub through which messages between stations are relayed, and is often connected to larger networks, such as the internet, and provides the stations with access to the larger network. The standards establish procedures for how the APs and STAs are to transmit information, and how they are to coordinate the use of the radio transmission medium. Coordination of when a device can use the radio transmission channel is known as medium access control (MAC).
The 802.11a/b/g/n standards also specify fixed channel bandwidths (20 MHz or 40 MHz with /n). They further specify the procedures for MAC, by which the use of a channel is coordinated within a basic service set (BSS) comprising a central AP and a set of stations STAs, e.g., laptops, cellphones, etc. They also specify the main frequency bands in which the channels are located (2.4 GHz (/b/g/n), 3.7 GHz (/a) and 5 GHz (/a/n)).
A fundamental feature of such networks is that an entire message is digitized and the digital data is organized into separate blocks and transmitted in frames. The frames include separate header fields carrying further information necessary for synchronization, network coordination, and reformulation of the message data. FIG. 1 shows two examples of the header field structure used in data and management frames in the 802.11 standards. The detailed terminology of frames and frame-based communications are specified in the standard IEEE 802.11-2012. The standard is cited as a reference for terminology and background information about frame transmission, and does not imply that the communication networks of this disclosure necessarily use the physical wireless transmission methods described therein.
Two recent, emerging amendments to the IEEE 802.11 standard (/ac and /ah) specify new frequency bands for transmission: respectively 5 GHz and Sub 1 GHz (902 MHz to 928 MHz). In the case of 802.11ac, the goal is to provide very high data rates (more than 500 Mbps), whereas for 802.11ah the goal is to provide long range (up to 1 km) for networks (e.g., smart grids or other sensor networks) with many (e.g., 6000) stations needing only low data rates (e.g., 100 kbps), on an intermittent basis. The 802.11 ah standard takes advantage of not needing to be backwards compatible with other standards, and so can optimize how the transmitted data is organized into frames, and optimize the content and organization of the header fields of the MAC and physical layer frames. The 802.11ac standard needs to be backwards compatible with the 802.11a/n standards.
Both standards specify that an entire allowed frequency range be subdivided into a fixed number of relatively narrow bandwidth channels of equal bandwidth, called fundamental channels, and that the devices can transmit using channels of different bandwidths, herein called transmission channels, built from multiple fundamental channels. When the duplicate frames are transmitted in transmission channels, each duplicate frame is transmitted in a fundamental channel. In the case of 802.11ac, the possible channel bandwidths are 20/40/80/160/80+80 MHz. In the case of 802.11ah, the channel bandwidths are 1/2/4/8/16 MHz. FIG. 3 shows the 802.11 ah channelization for the United States. The advantage of channels with wider bandwidths is greater data transmission rates. An advantage of the Sub-1 GHz frequency range is that it allows greater range, and suffers less interference from intervening objects.
However, having unfixed channel bandwidths available can create challenges for coordinating channel access. A first challenge is for the devices (APs or STAs) to have conflict-free transmission opportunities (TXOPs), in which only one device transmits at a time, and the various devices know the bandwidth and channels to be used by a transmitting device. In the 802.11ac standard, bandwidth information can be carried by PPDUs; duplicate frames transmitted in multiple fundamental channels are used for TXOP bandwidth indication and negotiation and for TXOP protection. The methods of 802.11ac need to be compatible with legacy 802.11a/n stations in the 5 GHz band. The fundamental channel bandwidth is 20 MHz since legacy 802.11a devices only understand 20 MHz PPDUs. Since 802.11a PHY SIG does not include bandwidth indication, 802.11 ac needs to put the bandwidth information in another place in the PHY header.
The difference between the 802.11ah standard and the 802.11ac standard is that the former does not require backward compatibility with older standards, and so new methods for TXOP bandwidth indication and negotiation methods can be deployed. The 802.11 ah standards can put bandwidth information in the PHY SIG subfield for bandwidth indication and bandwidth negotiation. It is not necessary that 1 MHz is the fundamental channel bandwidth. Another option is that 2 MHz is the fundamental channel bandwidth once 1 MHz devices can decode 2 MHz frames.